Aluminum etching



United States Patent Office 2,839,370 Patented June 17, 1958 ALUMINUM ETCHIN G Donald Gardner Foulke, Watchung, and Oscar Kendle Irgens, Rumson, N. J., assignors to Hanson-Van Winkle-Munning Company, a corporation of New Jersey N Drawing. Application May 4, 1956 Serial No. 582,662

9 Claims. (CI. 41-42) This invention relates to aluminum etching and, more particularly, to etching aluminum with an aqueous alkaline solution. The invention provides an improved process and an improved bath for etching articles of aluminum and aluminum base alloys over a wide temperature range.

During the fabrication of aluminum or aluminum base alloys, it is often necessary to etch the surface of the aluminum to provide a decorative finish or a base for further finishing. Immersion of the aluminum into a hot, alkaline solution, for example, results almost im- By its very formation, however, sodium aluminate causes several undesirable and adverse consequences to occur during the etching operation. As the concentration of sodium aluminate in the etching bath increases, there is a pronounced tendency for a hydrated alumina precipitate to deposit onthe Walls and heating coils of the etching tank. A decrease in the degree of hydration hardens this precipitate to a rock-like scale, which tenaciously clings to the tank walls and heating coils.

The appearance of this rock-like scale is due to the hydrolysis of sodium aluminate, produced by the etching process, to sodium hydroxide and aluminum hydroxide, as represented by the following equation:

In turn, the aluminum hydroxide, initially precipitated as a soft, more or less gelatinous mass, rapidly undergoes dehydration to yield the hydrated alumina:

localized heating of the hydrated alumina, precipitated on the tank walls and heating coils, causes it to harden to a rock-like scale.

As the quantity of this rock-like scale increases, the entire character and control of the etching operation alters. Not only does the scale deposit on the steam coils, rendering heat-transfer between the coils and etching solution more difficult and thereby overheating the coils but the scale frequently mars the surfaces of the aluminum article being etched. Removal of this hard mass can be accomplished only by draining the etching tank, and tediously hammering and chipping the scale from the drained tank and coils.

Using a conventional alkaline aluminum etchant bath,

we have found that this objectionable scale formation can be completely prevented by incorporating in such bath certain organic additives which either alter the character of the precipitate or prevent its decomposition to objectionable scale. We are not certain which of these effects is brought about by the additive compound, but its effect is to retain the precipitate in the form of a soft, easily removed sludge. Accordingly, the process of this invention for etching articles of aluminum and aluminum base alloys comprises bathing or immersing such articles in an aqueous alkaline altuninum etching bath containing at least 0.01 ounce per gallon of at least one primary carbinol of the group consisting of pentaerythritol, tetrahydrofurfuryl alcohol and 1,2,4- butanetriol. By adding these compounds to the etching bath, the precipitate forms and remains as a soft easily disposable sludge, thus permitting operation of the etching process over extended periods of time without danger of scale formation.

The process of our invention is accomplished most advantageously by using an aluminum etching bath in which the concentration of alkali is within the limits generally considered to be optimal for standard aluminum etching operations. Such etching baths usually contain from 2 to 16 ounces per gallon of sodium hydroxide, although quantities from /2 to 32 ounces per gallon of any alkali may be employed. With this etching bath, we generally prefer to employ from about 0.05 to about 0.5 ounce per gallon of the primary carbinol. When the etching operation is conducted while maintaining the bath at a temperature from about to about 212 F., it has been our experience that the concentration of sodium aluminate in the etching solution becomes fairly high before the soft sludge begins to form.

The particular carbinols used in accordance with this invention may be added with equal effectiveness to aluminum etching baths containing any of the alkalies or alkaline salts ordinarily used. Addition of pentaerythritol, tetrahydrofurfuryl alcohol or 1,2,4-butanetriol to baths containing various concentrations of such compounds as sodium hydroxide, trisodium phosphate and sodium carbonate results in the formation of a soft, microcrystalline, and occasionally flaky, sludge during the etching operation. Even upon immersing the aluminum articles in baths maintained at Widely dilferent temperatures, ranging from room temperature to 212 F., We have found no objectionable formation of scale nor any significant alteration in the soft character of the sludge produced.

The concentration of the particular primary carbinol additive employed is not critical, as long' as the amount present is not less than about 0.01 ounce per gallon. At concentrations below 0.05 ounce per gallon, there is sometimes a slight tendency for scale to form very slowly; the upper range is limited solely by the solubility of the additive in the bath. Generally we prefer to use concentrations of from 0.05. to 0.5 ounce per gallon, inasmuch as there is no special economic or operating advantage to be gained by employing higher concentrations.

To illustrate the applicability of the additives described in the invention, various concentrations of each of the primary carbinols, pentaerythn'tol, tetrahydrofurfuryl alcohol and l,2,4butanetriol, were added to an etching solution containing 6 ounces per gallon of sodium hydroxide, and an aluminum article was bathed or immersed in the resultant solution maintained at a temperature of F. Table I summarizes the character of the sludge obtained, in comparison with the rock-like scale formed when no additive was employed in the etching solution.

The concentration of sodium aluminate was determined at the time of incipient sludge formation. It is not understood why the use of pentaerythritol in the etching bath permits a greater concentration of sodium aluminate to accumulate prior to formation of a precipitate, than when either tetrahydrofurfuryl alcohol or 1,2,4-butanetriol are employed.

Although the basis or reaction mechanism by which these primary carbinol additives function is not understood, it is evident from the foregoing that incorporating one (or more) of them in an aluminum etching bath prevents the formation of rock-like deposits on the etching tank and heating 00118, and instead allows the dissolved aluminum to be easily removed in the form of a soft sludge.

We claim:

1. The process for etching articles of aluminum and aluminum base alloys which comprises bathing said articles in an aqueous alkaline aluminum etching solution containing at least 0.01 ounce per gallon of at least one primary carbinol of the group consisting of pentaerythritol, tetrahydrofurfuryl alcohol, and 1,2,4-butanetriol.

2. The process for etching articles of aluminum and aluminum base alloys which comprises bathing said articles in an aqueous alkaline aluminum etching solution containing from 0.05 to ounces per gallon of at least one primary carbinol of the group consisting of pentaerythritol, tetrahydrofurfuryl alcohol, and 1,2,4-butanetriol.

3. The process for etching articles of aluminum and aluminum base alloys which comprises bathing said articles at a temperature of about 100 to about 212 F. in

an aqueous alkaline aluminum etching solution containing from /2 to 20 ounces per gallon of an alkali metal hydroxide, and from 0.05 to 0.5 ounce per gallon of at least one primary carbinol of the group consisting of pentaerythritol, tetrahydrofurfuryl alcohol, and 1,2,4- butanetriol.

4. The process for etching articles of aluminum and aluminum base alloys which comprises bathing said articles at a temperature of about to about F.

in an aqueous alkaline aluminum etching solution containing from 2 to 16 ounce per gallon of sodium hydroxide, and from 0.05 to 0.5 ounce per gallon of pentaerythritol.

5. The process of etching articles of aluminum and aluminum base alloys which comprises bathing said articles at a temperature of about 150 to about 180 F. in an aqueous alkaline alumnium etching solution containing from 2 to 16 ounces per gallon of sodium hydroxide, and from 0.05 to 0.5 ounce per gallon of tetrahydrofurfuryl alcohol.

6. The process for etching articles of aluminum and aluminum base alloys which comprises bathing said articles at a temperature of about 150 to about 180 F. in an aqueous alkaline aluminum etching solution containing from 2 to 16 ounces per gallon of sodium hydroxide, and from 0.05 to 0.5 ounce per gallon of 1,2,4- butanetriol.

7. An aqueous alkaline aluminum etching bath containing at least 0.01 ounce per gallon of at least one primary carbinol of the group consisting of pentaerythritol,;tetrahydrofurfuryl alcohol, and 1,2,4-butanetriol.

8. An aqueous alkaline aluminum etching bath containing from 0.05 to 5 ounces per gallon of at least one primary carbinol of thegroup consisting of pentaerythritol, tetrahydrofurfuryl alcohol, and 1,2,4-butanetriol.

9. An aqueous alkaline aluminum etching bath containing from 2 to 16 ounces per gallon of sodium hydroxide, and from 0.05 to 0.5 ounce per gallon of at least one primary carbinol of the group consisting of pentaerythritol, tetrahydrofurfuryl alcohol, and 1,2,4- butanetriol.

References Cited in the file of this patent UNITED STATES PATENTS 2,146,839 Newsome et al Feb. 14, 1939 2,472,304 Mason June 7, 1949 2,650;875 Dvorkovitz et al Sept. 1, 1953 2,653,860 Meyer Sept. 29, 1953 

1. THE PROCESS FOR ETCHING ARTICLES OF ALUMINUM AND ALUMINUM BASE ALLOYS WHICH COMPRISES BATHING SAID ARTICLES IN AN AQUEOUS ALKALINE ALUMINUM ETCHING SOLUTION CONTAINING AT LEAST 0.01 OUNCE PER GALLON OF AT LEAST ONE PRIMARY CARBINOL OF THE GRUP CONSISTING OF PENTAERYTHRITOL, TETRAHYDROFURFURYL ALCOHOL, AND 1,2,4-BUTANERIOL. 